I've been using PHP for a number of years and have seen the same things being done with return values from functions over and over again. I have always thought of this as pretty standard, but the more I think about it the less it makes sense. Looking back over my career I am quite sure that a few serious bugs could have been avoided if I had not mixed return types.
As PHP is a loosely typed language this gives the developers the ability to change the type of value that is returned from a function. This happens quite often within the PHP codebase itself as many built in functions will return false if an error happened.
A common practice in userland code is to return false from a function if something went wrong. This might be because it is encouraged in PHP itself.